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B W Metcalf

Publications and source records attributed to B W Metcalf.

At least 37 records · Page 2Linked to original sources
PubMed1988

cis-4-Carboxy-6-(mercaptomethyl)-3,4,5,6-tetrahydropyrimidin-2(1 H)-one , a potent inhibitor of mammalian dihydroorotase.

A series of cis- and trans-4-carboxy-3,4,5,6-tetrahydropyrimidin-2(1H)-ones possessing either a carboxy, hydroxymethyl, or mercaptomethyl substituent at C-6 were prepared and tested for their ability to inhibit mammalian dihydroorotase. Of these compounds, only the cis-6-mercaptomethyl compound, cis-1, was found to be a potent competitive inhibitor of the enzyme (Ki = 140 nM at pH 7.4 and 8.5) when assayed in the direction of dihydro-L-orotate hydrolysis. These results suggest that the inhibition arises from the ligation of the thiolate to the zinc atom which is thought to be located in the enzyme's active site. Although analysis of cis-1 with 2,2'-dithiobis(5-nitrobenzoic acid) revealed significant loss of the free thiol group under enzymatic assay conditions, the addition of the reducing agent, dithiothreitol, to the enzymatic reaction mixtures afforded cis-1 complete protection against this chemical decomposition, as evidenced by lowering of the inhibition constant in the presence of dithiothreitol. Compound cis-1 had no significant antiproliferative activity against B16 melanoma cells in tissue culture, possibly due to the rapid decomposition of the compound or poor permeability into cells.

Amidohydrolases↗
PubMed1987

Inhibition of 3(17)beta-hydroxysteroid dehydrogenase from Pseudomonas testosteroni by steroidal A ring fused pyrazoles.

Several 2,3- and 3,4-steroidal fused pyrazoles have been investigated as potential inhibitors of NAD(P)H-dependent steroid oxidoreductases. These compounds are proven to be potent, specific inhibitors for 3(17) beta-hydroxysteroid dehydrogenase from Pseudomonas testosteroni with Ki values of 6-100 nM. In contrast, the activities of 3 alpha,20 beta-hydroxysteroid dehydrogenase from Streptomyces hydrogenans, steroid 5 alpha-reductase from rat prostate, and 3 alpha-hydroxysteroid dehydrogenase from rat liver were unaffected by micromolar concentrations of these compounds. Product and dead-end inhibition studies indicate an ordered association to the beta-dehydrogenase with the cofactor binding prior to substrate or inhibitor. From the results of double inhibition experiments, it is proposed that inhibition occurs through formation of an enzyme-NAD+-inhibitor ternate. On the basis of pH profiles of Vm/Km, Vm, and 1/Ki and of absorbance difference spectra, a hypothetical mechanism of inhibition by the steroidal pyrazoles, drawn by analogy from the inhibition of liver alcohol dehydrogenase by alkylpyrazoles [Theorell, H., & Yonetani, T. (1963) Biochem. Z. 338, 537-553; Andersson, P., Kvassman, J. K., Lindström, A., Oldén, B., & Pettersson, G. (1981) Eur. J. Biochem. 113, 549-554], is reconsidered. The pH studies and enzyme modification experiments by diethyl pyrocarbonate suggest the involvement of histidine in binding of the inhibitor. A modified proposal for the structure of the enzyme-NAD+-steroidal pyrazole complex is proposed.(ABSTRACT TRUNCATED AT 250 WORDS)

17-Hydroxysteroid Dehydrogenases↗
PubMed1987

Human immunodeficiency virus protease expressed in Escherichia coli exhibits autoprocessing and specific maturation of the gag precursor.

The mature gag and pol proteins of human immunodeficiency virus (HIV) and all retroviruses derive from large gag and gag-pol polyprotein precursors by posttranslational cleavage. A highly specific, virally encoded protease is required for this essential proteolytic processing. In this study, the HIV protease gene product was expressed in Escherichia coli and shown to autocatalyze its maturation from a larger precursor. In addition, this bacterially produced HIV protease specifically processed an HIV p55 gag polyprotein precursor when coexpressed in E. coli. This system will allow detailed structure-function analysis of the HIV protease and provides a simple assay for the development of potential therapeutic agents directed against this critical viral enzyme.

Amino Acid Sequence↗
PubMed1984

Time-dependent inhibition of aromatase in trophoblastic tumor cells in tissue culture.

Human choriocarcinoma trophoblast cells (JAr line) were utilized as whole cell preparations in tissue culture to evaluate the effects of mechanism-based inactivation or "suicide inhibition" of estrogen biosynthesis. A C-19 acetylenic analog (MDL 18,962) of the substrate, androstenedione, was evaluated in competitive and time-dependent assays. Product formation was determined by accumulation of 3H2O resulting from the stereo-specific elimination of 1 beta-tritium from the androgen substrate. Trophoblast cells exhibited initial linear kinetics for at least 3 h following addition of [1-3H]androstenedione. The Km for androstenedione was 35.1 nM with Vmax of 3.7 pmol/h/10(6) trophoblast cells. Kinetic analysis of time-dependent inhibition of aromatase in trophoblast cells revealed an apparent Ki of 0.6 nM for MDL 18,962 and at t 1/2 of inactivation of 26 min at infinite inhibitor concentration. These studies suggest that a suicide aromatase inhibitor can cause irreversible inhibition of estrogen biosynthesis in intact trophoblast cells. In the presence of 1 nM or 10 nM MDL 18,962, trophoblast cells exhibited initial linear kinetics for estrogen biosynthesis during the first hour following co-incubation with inhibitor and 33 nM substrate. During the subsequent 30 min the rate of estrogen biosynthesis precipitously declined from 104 +/- 24 fmol/min/10(6) cells in control cells to 24 +/- 13 and 8 +/- 4 fmol/min/10(6) trophoblasts treated with 1 or 10 nM MDL 18,962, respectively. This significant decrease in aromatase activity (P less than or equal to 0.01) implied irreversible inactivation, which was supported by prolonged inhibition of aromatase activity in trophoblast cells incubated for 6-48 h following removal of medium containing 3 nM or 30 nM MDL 18,962.

Androstenedione↗
PubMed1984

Biochemical and endocrine properties of a mechanism-based inhibitor of aromatase.

The conversion of androgens to estrogens by aromatase represents a major alteration in hormonal expression, and its regulation offers a promising method for therapeutic control of disease processes that are hormonally dependent. The design of suicide inhibitors based on enzyme-activated mechanisms provides an attractive approach for regulation of estrogen biosynthesis. MDL 18,962, (10-2(-propynyl)estr-4-ene-3,17-dione), a C-10 substituted analog of the natural substrate androstenedione, was evaluated as a suicide inhibitor of aromatase. Appropriate kinetic evaluations of MLD 18,962 established it to be a highly potent [inhibition constant (Ki) = 4.5 +/- 1.3 nM] irreversible inhibitor of human placental aromatase. The 2-propynyl group was necessary for time-dependent inactivation, as this activity was lost in related compounds. The inactivation process was specific for aromatase, since other P450 enzyme systems are not inactivated by MDL 18,962. This compound exhibited minimal intrinsic hormonal properties, since weak binding affinities were observed for cytosolic androgen, estrogen, or progestin receptors. The stimulation of ovarian aromatase activity by gonadotropins in immature mice was inhibited in animals implanted with 10-mm MDL 18,962 Silastic implants. This inhibition of estrogen biosynthesis suppressed estrogen-dependent uterine growth in these mice.

Androstenedione↗
PubMed1983

(2R,5R)-6-heptyne-2,5-diamine, an extremely potent inhibitor of mammalian ornithine decarboxylase.

It was previously shown that 5-hexyne-1,4-diamine is a potent enzyme-activated irreversible inhibitor of mammalian ornithine decarboxylase. However this compound has secondary pharmacological effects owing to its in vivo oxidation to 4-aminohex-5-ynoic acid, an irreversible inhibitor of 4-aminobutyrate aminotransferase. The first step of this oxidation is catalysed by mitochondrial monamine oxidase. The monomethyl and dimethyl analogues of 5-hexyne-1,4-diamine, i.e. 6-heptyne-2,5-diamine and 2-methyl-6-heptyne-2,5-diamine, which cannot be substrate of monoamine oxidase, were tested as selective irreversible inhibitors of ornithine decarboxylase. Our results demonstrate that (2R,5R)-6-heptyne-2,5-diamine is greater than 10 times more potent, both in vitro and in vivo, than alpha-difluoromethylornithine, the most widely used irreversible inhibitor of this enzyme.

Alkynes↗
PubMed1981

alpha-Ethynyl and alpha-vinyl analogues of ornithine as enzyme-activated inhibitors of mammalian ornithine decarboxylase.

alpha-Ethynyl- and alpha-vinylornithine were designed and synthesized as potential enzyme-activated inhibitors of mammalian ornithine decarboxylase. These two new inhibitors produce both immediate and time-dependent inhibition of rat liver ornithine decarboxylase in vitro. The inhibitions exhibition saturation kinetics. The apparent dissociation constants (KI) are 10 and 810 microM, and the times of half-inactivation at infinite concentration of inhibitor (t1/2) are 8.5 and 27 min, respectively, for alpha-ethynyl- and alpha-vinylornithine. In rats, alpha-ethynylornithine causes a rapid dose-dependent decrease of ornithine decarboxylase activity in prostate and, to a lesser extent, in thymus and testis.

Animals↗
PubMed1978

Mechanism of the stereospectific irreversible inhibition of bacterial glutamic acid decarboxylase by (R)-(--)-4-aminohex-5-ynoic acid, an analogue of 4-aminobutyric acid.

4-Aminohex-5-ynoic acid inhibits bacterial glutamic acid decarboxylase in a time-dependent irreversible manner. The inhibition is stereospecific and requires the abstraction of the propargylic hydrogen from 4(R)-(--)-4-aminohex-5-ynoic acid. This leads to the generation of a reactive alkylating agent in the active site which can react with a nucleophilic residue. At complete inhibition, there is incorporation of one molecule of inhibitor per pyridoxal binding site. If the decarboxylation of glutamate occurs with retention of configuration, the irreversible inhibition of this enzyme by the 4-(R) isomer can be rationalized on the basis of reversibility of the protonation step in the normal catalytic mechanism.

Aminocaproates↗
PubMed1977

4-amino-hex-5-enoic acid, a selective catalytic inhibitor of 4-aminobutyric-acid aminotransferase in mammalian brain.

Incubation of rat brain 4-aminobutyrate aminotransferase with 4-amino-hex-5-enoic acid, a substrate analog of 4-aminobutyric acid, results in a time-dependent irreversible loss of enzymatic activity. In the presence of 0.1 mM inhibitor the half-life of the inactivation process is approximately 6 min. Low concentrations of L-glutamic acid or 4-aminobutyric acid protect against this inactivation, while 2-oxoglutarate prevents this protection, suggesting that only the pyridoxal form of the enzyme is susceptible to inhibition by 4-amino-hex-5-enoic acid. The irreversible inhibition of mammalian 4-aminobutyrate aminotransferase by 4-amino-hex-5-enoic acid is selective. There is no inhibition of this enzyme from Pseudomonas fluorescens with the inhibitor at mM concentrations. Even at 10 mM there is no irreversible inhibition of mammalian glutamate decarboxylase or of aspartate aminotransferase, while alanine aminotransferase is inhibited over 500 times more slowly than rat brain 4-aminobutyrate transaminase.

4-Aminobutyrate Transaminase↗